The present invention relates generally to ophthalmic instruments, and more particularly to subjective ophthalmic refractors for evaluating refractive characteristics of a patient""s eye.
A subjective ophthalmic refractor typically comprises left-eye and right-eye batteries each having a defined viewing path along which an operator may selectively introduce combinations of testing lenses having known refractive properties. During examination, the patient is positioned in a darkened room with his or her eyes aligned to view a projected target chart along the viewing paths defined by the left-eye and right-eye batteries. The operator then performs well-known refracting procedures, including refraction using astigmatic charts and the Jackson cross-cylinder test. A goal of the examination procedure is to determine the sphere power, cylinder power, and cylinder axis of each eye so that a suitable pair of corrective lenses may be prescribed.
U.S. Pat. No. 2,968,213 describes an ophthalmic refractor of the prior art. FIG. 5 of the ""213 patent is an exploded view illustrating the internal components of a left eye lens battery, and serves to illustrate a lonstanding arrangement for mechanically coupling a pair of rotatable cylinder lens carriers 8 and 9 and an associated ring-shaped cylinder power scale 50 of the battery to an adjustment knob 20 used by the operator to set a chosen cylinder power in the viewing path of the battery. As can be seen at FIG. 1 of the ""213 patent, the indicia on scale 50 can be viewed by the operator through an opening 52 in the battery housing. Typically, the indicia are numerical cylinder power values from 0 to 6.00 diopters in quarter-diopter increments, and are angularly spaced at regular angular increments about a central axis of the ring-shaped scale. The mechanical interconnections from knob 20, through cylinder power scale 50, to the lens carriers 8 and 9 are designed such that rotation of knob 20 positions two lenses (or a lens and an empty lens cell), one from lens carrier 8 and one from lens carrier 9, in series in the viewing path to produce a resultant cylinder power. Lens carrier 8 is a xe2x80x9cweakxe2x80x9d cylinder lens carrier having, for example, a blank lens cell (zero power) and four cylinder lenses ranging in power from 0.25 diopters to 1.00 diopters at quarter diopter increments. Meanwhile, lens carrier 9 is a xe2x80x9cstrongxe2x80x9d cylinder lens carrier having, for example, a blank lens cell (zero power) and four cylinder lenses ranging in power from 1.25 diopters to 5.00 diopters at 1.25-diopter increments. Consequently, by indexing the weak cylinder lens carrier 8 five times for every one index movement of strong cylinder lens carrier 9, a cylinder power range of 0.00 diopters to 6.00 diopters at quarter-diopter increments is possible in agreement with the indicia on scale 50.
With continued reference to U.S. Pat. No. 2,968,213, it will be seen that adjustment knob 20 drives a shaft 21 having at its opposite end a dual gear comprising a small front gear 23 and a larger rear gear 25. Larger gear 25 meshes with a gear 31 fixedly connected to the hub of a driver plate 27, whereby rotation of knob 20 and larger gear 25 produces counter-rotation of driver plate 27. As driver plate 27 rotates about its axis, four short pegs 39a-39d on the driver plate successively engage weak cylinder lens carrier 8 to index only the weak lens carrier, and a fifth longer peg 40 engages both the weak and strong lens carriers 8 and 9 to index both carriers, in the manner of a Geneva mechanism. A spring-biased roller 48xe2x80x2 cooperates with five circumferential detents 53xe2x80x2 in driver plate 27 to allow the operator to feel each index position at adjustment knob 20.
Meanwhile, small front gear 23 meshes with internal gear teeth on ring-shaped scale 50 to rotate the scale in coordination with the indexing of cylinder lens carriers 8 and 9, whereby an appropriate cylinder power value marked on the scale appears through opening 52. Scale 50 is constrained both radially and axially by three bearings 56 located at respective positions about the circumference of scale 50. Each bearing 56 includes a low friction circular base slidably contacted by the marked side of scale 50, and a stepped cylindrical retainer post fastened coaxially along with the base to the refractor housing to define a gap through which the circumferential outer edge of scale 50 passes. Due to space limitations within the refractor housing, bearings 56 are not spaced at regular 120xc2x0 angular intervals about the rotational axis of scale 50. This reliable arrangement for rotatably supporting and positioning scale 50 has heretofore remained unimproved, despite the existence of certain drawbacks with respect thereto. In particular, bearings 56 are difficult to adjust during assembly, the tolerances of the retainer posts and scale 50 cause misalignment of indicia in view opening 52, and the localized support at irregular angular intervals creates a poor fit. Consequently, the rotation of adjustment knob 20 does not feel as smooth and uniform to the user as would be desired.
Therefore, it is an object of the present invention to provide an improved arrangement for mounting a rotatable ring-shaped scale in an ophthalmic refractor that allows for easy assembly.
It is another object of the present invention to provide an improved arrangement for mounting a rotatable ring-shaped scale in an ophthalmic refractor that achieves a smoother adjustment knob feel to the user without introducing more demanding tolerances in critical components.
It is a further object of the present invention to provide an improved arrangement for mounting a rotatable ring-shaped scale in an ophthalmic refractor that reduces the overall number of bearing components.
In furtherance of these and other objects, an ophthalmic refractor of a type comprising a ring-shaped cylinder power scale having an internal gear, a rotatable cylinder power adjustment knob, and a scale drive gear fixed for rotation with the adjustment knob and meshing with the internal gear of the cylinder power scale is improved with respect to the manner in which the cylinder power scale is supported and located. In particular, a prior art arrangement of three bearing posts irregularly-spaced at points on the outer circumference of the cylinder power scale is replaced by a generally circular bearing insert located in the interior of the scale and mounted on an existing axle stem fixed to the an eye battery housing of the refractor. The cylinder power scale is confined against radially directed movement by a first retaining surface of the bearing insert that faces in a radially outward direction for engaging the crests of the scale""s internal gear teeth. The cylinder power scale is confined against axially directed movement by a second retaining surface defined by a circumferential lip on the bearing insert, and by a low-friction bearing plate fixed to the eye battery housing. A U-shaped cut-out region in the bearing insert allows space for the scale drive gear. The arrangement of the present invention provides more uniformly and extensively distributed bearing surfaces for guiding the gear-driven rotation of the cylinder power scale in an improved manner.